Tipping Points in the Earth’s Climate

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Tim Lenton has said on his Website (http://researchpages.net/ESMG/people/tim-lenton/tipping-points/) that, “The Intergovernmental Panel on Climate Change (IPCC) in its many excellent reports tends to portray climate change as a smooth transition, although it acknowledges the non-linear nature of the Atlantic thermohaline circulation (THC) system and the Greenland ice sheet (GIS). Although the projections are rarely straight lines the underlying system and its responses appear ‘linear’ (in mathematical terms)”, he adds that “there are many components (or sub-systems) of the Earth system that could display non-linear behaviour and transitions under human (anthropogenic) climate forcing.”

Tim Lenton also referred on his Website to previous events in the Earth’s history where a “tipping point” effect appears to have happened, such as the Paleocene-Eocene Thermal Maximum (PETM) which occurred fifty-five million years ago, at that time there is a striking warm spike in the global temperature record. Tim says that “This provides probably the closest available analogue to human impact on the climate system. Around 1500–4500 PgC of ‘fossil’ carbon was released – a similar amount to known fossil fuel reserves today. This carbon may have come from frozen methane hydrates under the ocean sediments or from a volcanic intrusion into a huge fossil fuel reservoir where the North Atlantic was opening up – we don’t know for sure. We do know that within the resolution of the rock record (thousands of years), temperature at the equator rose 4–5 °C and at the poles 8–10 °C. The ocean acidified dissolving carbonate sediments and the system took the order of a hundred thousand years to recover. If, as many think, much of the carbon released came from frozen methane hydrates after they received a small initial perturbation, then they were a tipping element at the time.” He also refers to the relevantly recent (5,000 years ago) non-linear transition, when much of the Sahara switched from being vegetated to the present desert and to the Bölling-Allerød warming during the last deglaciation when temperatures recorded in Greenland ice samples show a rapid change in temperature.

Given the IPCC figures Tim predicts that “Our present warming commitment alone seems insufficient to tip any of the elements we have identified. However, it could get us close to the threshold for irreversible melt of the Greenland ice sheet. If that threshold is at the nearest end of its estimated error range (1 °C further global warming) then it will be nearly impossible to avoid by mitigation unless we are lucky and the climate sensitivity is at the bottom end of its uncertainty range (circa 1.6 °C warming for a doubling of pre-industrial CO2).” The Greenland ice sheet is predicted to take at least 300 years to melt, and the estimate is often given as roughly 1000 years. But the Greenland ice sheet contains 7m of global sea-level rise, and this could impact the melting of the West Antarctic Ice Sheet (WAIS), Tim says, “A worst case scenario is for collapse to occur within 300 years, with a total of 4–6m of global sea-level rise”, and the melting of the GIS could also affect the stability of the Atlantic thermohaline circulation system (The Gulf Stream). It seems that we need to plan to relocate all coastal settlements which are between 1 and 13 metres above sea level, now, because over 300 years the melting of the Greenland ice sheet and the possible melting of the West Antarctic Ice Sheet could raise sea level by a total of 13 metres, and this is without taking into effect the melting of continental glaciers.

In the article, which is available on www.pnas.org, the authors identify a number of possible “tipping elements”:

Arctic summer sea-ice
Greenland ice sheet
West Antarctic ice sheet (WAIS)
Atlantic thermohaline circulation (THC)
El Nino–Southern Oscillation (ENSO)
Indian summer monsoon (ISM)
Sahara/Sahel and West African monsoon (WAM)
Amazon rainforest
Boreal forest
Antarctic Bottom Water (AABW)
Tundra
Permafrost
Marine methane hydrate (MMH)
Ocean anoxia
Artic ozone
The article the authors highlight the problems of Artic summer sea-ice loss, they say that “Given that the IPCC models significantly underestimate the observed rate of Arctic sea-ice decline, a summer ice-loss threshold, if not already passed, may be very close and a transition could occur well within this century.” With regard to the Greenland ice sheet they say that, “warming at the periphery lowers ice altitude, increasing surface temperature and causing a positive feedback that is expected to exhibit a critical threshold beyond which there is ongoing net mass loss and the GIS shrinks radically or eventually disappears. During the last interglacial (the Eemian), there was a 4- to 6-m higher sea level that must have come from Greenland and/or Antarctica.” The authors point out that the loss of Arctic sea-ice will warm Greenland and that “recent observations show the surface mass balance is declining and contributing to net mass loss from the GIS that is accelerating. Finally, existing ice-sheet models are unable to explain the speed of recent changes. These changes include melting and thinning of the coastal margins and surging of outlet glaciers.” These factors make them suggest that a lower limit of 300 years is conceivable for the melting of the Greenland ice sheet.

Regarding the West Antarctic ice sheet (WAIS), the authors say, “Although the timescale is highly uncertain, a qualitative WAIS change could occur within this millennium, with collapse within 300 years being a worst-case scenario. Rapid sea-level rise (over 1 m per century) is more likely to come from the WAIS than from the GIS.” The British Government has been extremely concerned about possible changes to the Atlantic thermohaline circulation (THC) which could result in the warm waters of the Gulf Stream being diverted south away from the British Isles and N.W. Europe. The authors say that, “the IPCC does not include freshwater runoff from GIS melt. Subsequent OAGCM simulations clearly pass a THC tipping point this century and undergo a qualitative change before the next millennium.” In other words they do not accept the the IPCC’s that assertion that an abrupt transition of the THC is “very unlikely” to occur before 2100.

The article suggests that past climate changes have been accompanied by changes in the El Nino–Southern Oscillation (ENSO), “we differ from IPCC and consider there to be a significant probability of a future increase in ENSO amplitude. The required warming can be accessed this century.” The authors also point to the potentially chaotic nature of the Indian Summer Monsoon (ISM), they say that “under some plausible decadal-scale scenarios of land use and greenhouse gas and aerosol forcing, switches occur between two highly nonlinear metastable regimes of the chaotic oscillations corresponding to the ‘‘active’’ and ‘‘weak’’ monsoon phases.” In other words there is a possibility that the ISM may rapidly change and become unstable, this would have disastrous consequences for India, the second most populous nation, and for the other nations of South Asia. One rare example of a possibly beneficial development caused by abrupt change is the likelihood of the greening of the Sahara/Sahel.

Generally predictions for the future of the Amazon fainforest have been dire; the authors of this article note that a regional climate model predicts Amazon dieback and increases in forest fires, they say that, “the fate of the Amazon may be determined by a complex interplay between direct land-use change and the response of regional precipitation and ENSO to global forcing.” The article notes that there are uncertainties in understanding the effects of temperature increases on the Boreal Forest. They did not consider the final six items on the above list because they saw their effects as being long, or medium term (after 2100).

The authors conclude by saying, “Society may be lulled into a false sense of security by smooth projections of global change. Our synthesis of present knowledge suggests that a variety of tipping elements could reach their critical point within this century under anthropogenic climate change. The greatest threats are tipping the Arctic sea-ice and the Greenland ice sheet, and at least five other elements could surprise us by exhibiting a nearby tipping point.” Finally they say, “there is an urgent need to improve our understanding of the underlying physical mechanisms determining their behavior, so that policy makers are able ‘to avoid the unmanageable, and to manage the unavoidable’.”

This is a very important article and by reassessing much recent work on possible abrupt climate change events it goes beyond the IPCC’s 4th Assessment Report, in particular the authors make the case for thinking that the risk from sea-level rise is much greater than is generally believed. If the Greenland ice sheet completely disappears in 300 years, increasing global sea level by 7 metres, AND we see large amounts of water being added to the oceans by the West Antarctic ice sheet, then total sea-level rise by 2100, or even 2050, will be significant, and the coastal mega-cities of the third world have no long term future in their current locations; Shanghai, Lagos, Bangkok, Jakarta, Saigon, Bombay, Calcutta, are all threatened, not to say many of the port cities of the developed world, Amsterdam, London, New York, Los Angeles and Tokyo.